The present invention relates to a method of driving a display device comprising an electro-optical display medium between two supporting plates, a system of picture elements arranged in rows and columns with each picture element being constituted by picture electrodes provided on the facing surfaces of the supporting plates and a system of row and column electrodes, a row of picture elements being selected during a selection period by applying a voltage thereto via the row electrodes by means of non-linear switching elements arranged in series with the picture elements while data signal is presented thereto via the column electrodes.
The invention also relates to a display device in which such a method can be used.
In this respect it is to be noted that the terms row electrode and column electrode in this application may be interchanged if desired, so that a column electrode can be meant where reference is made to a row electrod while simultaneously changing column electrode to row electrodes.
A display device of this type is suitable for displaying alpha-numeric and video information with the aid of passive electro-optical display media such as liquid crystals, electrophoretic suspensions and electrochrome materials.
A display device, as described, is shown in U.S. Pat. No. 4,223,308 in which back-to-back diodes are used as switching elements. By using switching elements a memory action is obtained so that the information presented to a driven row remains present across a picture element to a sufficient extent during the period when the other row electrodes are driven. However, due to capacitive cross-talk caused by the capacitance of the non-linear switching elements this information may have a varying value because the same columns are used for presenting data signals during selection of different rows of picture elements.
The voltage across a picture element may then vary in such a way that the transmission level shifts to a higher or lower degree of transmission (grey level). If the grey levels are to be fixed exclusively via the transmission curve, the number of grey levels is limited to a large extent due to the said crosstalk in relation to the maximum signal level.
The crosstalk due to signal variations is in the first instance dependent on the capacitance of the non-linear switching elements.
Another possibility of realizing grey levels is to subdivide a picture element into a number of subsections sub-sections determines the grey level. This requires an extra drive with extra column electrodes.
Such a sub-division without extra drive may also be used for the purpose of providing a given redundancy, because connections may drop out. This sub-division usually leads to smaller sub-elements for which smaller picture electrodes are used. However, this results in the capacitance of the picture elements decreasing (relatively) with respect to that of the non-linear switching elements. As a result the said crosstalk becomes larger.